A multi-wavelength study to decipher the 2017 flare of the blazar OJ 287

TL;DR

This study analyzes the 2017 multiwavelength flare of blazar OJ 287, revealing complex emission behavior, a VHE gamma-ray outburst, and evidence for multiple emission zones, including a recollimation shock outside the radio core.

Contribution

It provides a detailed multiwavelength analysis of the 2017 flare, identifying multiple emission zones and linking the flare to a recollimation shock outside the radio core.

Findings

Detection of VHE gamma-ray emission above 100 GeV with 10σ significance.

Identification of at least two non-thermal emission zones during the flare.

Evidence linking the flare to a recollimation shock outside the radio core.

Abstract

In February 2017, the blazar OJ~287 underwent a period of intense multiwavelength activity. It reached a new historic peak in the soft X-ray (0.3-10 keV) band, as measured by Swift-XRT. This event coincides with a very-high-energy (VHE) $\gamma$-ray outburst that led VERITAS to detect emission above 100 GeV, with a detection significance of $10\sigma$ (from 2016 December 9 to 2017 March 31). The time-averaged VHE $\gamma$-ray spectrum was consistent with a soft power law ($\Gamma = -3.81 \pm 0.26$) and an integral flux corresponding to $\sim2.4\%$ that of the Crab Nebula above the same energy. Contemporaneous data from multiple instruments across the electromagnetic spectrum reveal complex flaring behavior, primarily in the soft X-ray and VHE bands. To investigate the possible origin of such an event, our study focuses on three distinct activity states: before, during, and after the February 2017 peak. The spectral energy distributions during these periods suggest the presence of at least two non-thermal emission zones, with the more compact one responsible for the observed flare. Broadband modeling results and observations of a new radio knot in the jet of OJ~287 in 2017 are consistent with a flare originating from a strong recollimation shock outside the radio core.